Radio navigation system



Oct. 13, 1953 H. J. scHRADER RADIO NAVIGATION SYSTEM Filed Deo. 51. 1948 Patented Oct. 13, 1953 RADIO NAVIGATION SYSTEM Harold John Schrader, Haddonfeld, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1948, Serial No. 68,644

8 Claims.

This invention relates to improvements in radio navigation systems wherein ships or aircraft receive information transmitted over a television link from a ground radar.

Three such systems are described in co-pending U. S. patent applications Serial No. 613,509, filed August 30, 1945, by Irving Wolff, now Patent No. 2,528,202, issued October 31, 1950; Serial No. 607,9991/, filed July 3l, 1945, by Loren F. Jones, now Patent No. 2,632,157; and Serial No. 725,206, filed by Philip J. Herbst on January 30, 1947, now Patent 2,490,268, issued December 6, 1949.

In general the improvements of the present invention prevent false self-identification, improve the picture quality of a self-identification" line which forms part of the plan-position display produced at each television receiver, and prevent the display of unwanted trafiic information. More particularly these ends are accomplished in part by adding to a self-identification circuit, which in the prior art responds to any radar interrogation to intensify the plan-position display in order to show the self-identification line, a coincidence circuit which limits its responsiveness to interrogations by the radar at the particular ground station which is in television contact with the craft. The coincidence circuit is actuated by the interrogating radar pulses and certain ones of the television synchronization pulses which are made to be synchronous therewith; and in part by separating in the television receiver signals which represent traffic information from signals which represent the` self-identication line. The latter expedient prevents multi- -plexing of the traffic-information signals from lowering the frame rate of the self-identification line signals and prevents the operation of the self-identification circuit from interfering with the visual presentation of traflic-information signals.

In accordance with the systems of the kinds described in the above-mentioned applications a ground radar set, assisted by beacon transponders carried in aircraft, produces information pertaining to air traflic conditions in its operating `areas.

adapted to characterize its responsive signals in accordance with which one of a number of predetermined altitude layers is occupied by the craft. for example, the carrier frequency of the responses may be variable stepwise under control of the altimeter, or as in the system described in the application Serial No. 725,206, now Herbst Patent 2,490,268, the responses may consist of pairs of pulses and be characterized ac- The transponder beacon in any craft is cording to altitude by controlling the spacing between them. Whatever type of characterization is used by the beacon, appropriate equipment is provided in the ground radar for separating the different beacon responses according to the altitude layers from which they come so that the air traiiic information is usefully subdivided. Radar signals representing the subdivisions of information may be fed to individual indicators to produce a plurality of plan position displays suitable for televising or, instead, they may be more directly translated into television type of electrical signals by the use of storage cathode ray tubes having semi-conducting targets.

At the ground station the subdivisions of air trailic information, having been appropriately translated into television type signals, are broadcast to the aircraft, each of which has a television receiver for translating the signals into a plan-position display for any desired altitude stratum. In some systems the television transmitter employs a plurality of carrier frequency channels, each one being allotted for broadcasting a different subdivision of the traffic information. However, inasmuch as this requires a very large overall frequency band, an improved system was developed which uses time channel multiplexing (see the co-pending Herbst application Serial No. 725,206 mentioned above).

As shown in the Wolff application Serial No. 613,509 and the Jones application Serial No. 607,9991/2, the system may also comprise means in each craft for identifying which pip represents that craft in the plan position display for its altitude layer. A preferred form of this means comprises a circuit in the beacon receiver in each aircraft for integrating radar pulse energy which it receives during each interrogation to produce a gate pulse, and a connection to the associated television receiver for increasing its sensitivity with this pulse. This causes a self-identification line, which marks a particular pip, to appear on the plan position display-beeause of the following: The composite television signal for each altitude level is caused to include components representing a line which rotates in synchronism with the radar antenna, for example, the signal may include components derived by televising, as part of a plan-position display at the ground radar, the time-base strobe line which is made to appear therein by setting the gain of the ground radar receiver suiciently high to cause the fluorescent screen of the radar indicator (which is producing that display) to be excited by random noise transients. Or, as i1- sioned once for every interrogation when tlie""'lv sensitivity of the receiver is transientiyyincreasedi. Prior art navigation systems which make use Y of a self-identification line' have liada. di'sadf-Q" vantage which arises frorrrthe "fact'that ferent ground radars will normally employ the same radar carrier frequencyl'-"-e-venlt'liouglithemr may employ different television carrier frequencies.

idetication line which appears'lonkthe planposition displ'ayis unsatisfactoryi 'Assuming fthe-:use ofl as few` asfve' altitudestrata'and of aseman'y television framesper Seclf'l'dfas 45'th'e'`fnnlr of frames use`f1'1`ll'y displayed'vinanycraf-t'iwill only befninep'er second. By using-a1persistent'phospho: the? television picture 4tulc'e flicker o'fpps prsented' inthedisplay can almost entirely The eliminated.; However, it' is notv this ea-sy to pro'- ducea' steadyVY display-of' the self-identification line.y If theradar hea-InY is` one d'egreef wide and theiaziinuthalscahning rate-"12` per minute, there will ltie'onlyA one* interrogation every five seconds andfi will last only about" 14 milliseconds. Each very' 'occasional and' brief Ainterrogations whichrd'oesnzot happen tofocc'ur during the" one particularfram'ei out of iivewhichis theoniy one usefiillyz displayed,v will'v contribute Anotl'ring t'owa'rd`"'brighteningV the' television picture: -I have found thatbecauseof this thev marker-line app'erslut occasionallyandin a: non-periodic znamierV y 'I'her'eis 'ai further disadvantage for particular prior-fart' 'errrhodirnents of navigation systems in whichA bothlthe' 'above-mentioned self-identificatiofriarndI tme'channel multiplexing are employed. In' these-particular j embodiments,l the unwanted informationreaching ari-aircraft' in other' time kchairlinels than that' selectedto he displayed is reject'ed` By a gatingci-rcuit which lowersthe receifver'j'sensitivity` during those time channels, Sincethe self-identification circuit -caus'esihe receiver-'sensitivity to be increased; I have found that an interrogation which `happens 'to occur Ad'urirfg an unwanted time channel: nia-yf oppose As a result, interrogations by'otlier radarsthan the one in television contact with an u 4 produced as a result of interrogations of some other radar.

It is a further object of the present invention to devise an improved system of the kind set forth in which the picture quality of the selfidentification line will be improved.

It is a further object of the present invention to'V devise an improvedi system:i of vtha hind set forth in which'the self-identicationv circuit will not operate to actuate a plan-position indicator "with signals broadcast in an undesired time chan- Vnelrandirepresenting information which is intended to be suppressed.

Other objects,v"avantages and features of this invention'lwill Beapparent to those skilled in the art` from the following detailed description of an illustrative-embodiment of the present invention and fromthe drawing in which the single figure represents' an improved radio navigation system according to the presenti/invention.

f In thefdiawingitherefare shown' agrounradar If cooperating i'with an: airborne: transponder? beacon-2 to 'obtaine'datarepresenting: thei posi,- tion of the craft (not show-n) carryingthe beacon; It: i'sto bei understoo'df that: in. the operationrof thejsys'tem herein, there usuallyi will be a" plue ralityvof/ aircra'ftf inlthe. operational area; served byth'eflradar andi that: therefore there; willbm a pluralityoff airborne` transponder; beacons1cooperating-2 With= 'thei'ground radar` set; each one doing sof-inf theV manner: shown inthe,2 singlei .exi-1f. ampl'evherein..

Asl is-.fullydescribed fin theme-pending, applicationsi'citedfherein, the' groundfiradarzset -Iz come prises-Lia. directive antenna 3: which is. usedfboth for' 'transm'ittin'g-outgoing radarl pulses-.anch for receivingincoming: echoes'. thereof arrdfor re;- s'pons'e'swfr-om beacons. vTo this end,. a. 'Ll-R box 4= usedtinfa knownf manner fon connecting the antenna-13 to1 bothfaradar transmitter-5 and*y radar receiverv 6 i modulatorrL comprisingmeansrfor periodical yf. producing: extremely :short-pulses. isconnected? to: key the trlansrnitterf 5; so thatsth'e transfMesi-'orma` fromzthe antenna :3; on the; form ofE periodically y repeated .-"rshort burstsg. fof. electromagnetic:ener-gm'v According to" the: present invention feach: of thev radar-pulses 'istransmittedf in.. synchronism Withone offrithei'highrrequency synchronization pulsesotfth" composite televisiontsignalf broad;- castaf'rom. the television :transmitter lasso:-

i zation signalsaris; coupled to'tlie rinodulator- Hover a;l synchronization cuit'- Inlshownl hereinlasasingle line. ThiszrepiresentationI vvis only-diagrammatic andthe; circuit itself'l woul'd 'not bei so 'simplegfor example,` even the simples electrical V`connection1between-the generatorl 9" andithe' v.modulator "l would include two conductors. Moreover, the circuit I0? might evenlcompriseanadditionall pulse generator feedabove-mentioned co-pending Herbst application Serial No. 725,206. Television transmitter 8 comprises a group of signal translators I2, each adapted to translate signals reaching it from one of the altitude selectors, from their form as radar signals to the form of television signals representing the same subdivision of the traffic information. There are as many translators as altitude selectors. In the drawing, the three lines I3, I4 and I5 represent circuits interconnecting the respectively cooperating altitude selectors and translators.

Each of the translators may consist of a radar indicator tube for producing a plan-position display of the subdivision of traffic information selected for it by the action of its associated altitude selector and a cooperating television pickupdevice, such as a storage orthicon, for converting the display into television type signals. If a storage orthicon is employed, the indicator may have the short-persistence screen and an effect corresponding to persistence will be simulated by the storage pick-up tube.

However, it is not essential that the radar signal reaching a translator be converted into a visible display. Instead, each translator, i.'e., each combination of a picture tube and a pick-up device, may be replaced by a single cathode ray tube of a kind in which a storage type target (which ordinarily is not fluorescent) has the subdivision of trailic information Written onto it, as a charge pattern, by a beam scanned in accordance with the pulsing of the radar and the rotation of its antenna 3, and has the charge pattern read by a beam scanned in accordance with the television raster to translate it into a television type of a signal. Obviously, either form of translator requires a means for scanning of a Writing cathode ray beam in accordance with the pulsing of the radar and the rotation of the radar antenna. Accordingly, in the arrangement shown herein a motor I6, which serves to drive the antenna 3 for azimuthal scanning, is connected to the group of translators over a mechanical link I1 and a branch link I8. The element of each translator controlled over the mechanical linkage may consist of a rotatable magnet (not shown) of the kind which is wellknown and Widely used for producing plan position displays. The group of translators I2 may be provided with a common sweep generator or a group of sweep generators feeding the rotatable magnets to produce periodic currents having sawtooth wave forms. It is to be understood that any such generator will be synchronized with the radar over a synchronization line not shown.

Included along with the translators in the circuits represented by block I2 of the drawing is a commutator circuit by which the respective translators are sequentially connected to a single output conductor I9 in a predetermined order so that their outputs are assigned to different time channels. In the Herbst patent application Serial No. 725,206, such a commutator device is shown as a multivibrator ring oscillator having a plurality of stages which operate successfully to turn on different ones of the pick-up tubes comprised in the group of translators used therein so that at any one time only one of them is delivering its output signals to a common output circuit. Frame synchronizing pulses are applied to the commutator device from the generator 9 causing it to move one step at a time. Associated with one stage of the commutator device isr a 6 pulse generator (not shown) providing a timing mark at the conclusion of each sequence, i. e., a time reference for the periodically repeated time channels.

The cooperation of the group of altitude selectors II and the group of translators I2 herein is generally as follows: The group of altitude selectors I I will receive in parallel from the radar receiver 5 intermingled pulse responses arriving from beacons at various altitudes, each of the pulse responses being characterized according to one of the predetermined altitude strata; each of the altitude selectors Will accept only the responses characterized according to a particular altitude layer and will route those signals over a different one of the circuits I2. I4 or I5 to one of the translators associated with it; each translater will translate the radar signals which it receives into corresponding television signals, and by the action of the commutator circuit associated With the group of translators, it will deliver them onto the common output conductor I9 in a predetermined time channel assigned to it. The thus multiplexed signals pass from the group of translators over output conductor I9 to a mixer 20. These signals will be referred to as televisiontype signals representing subdivision of traiiic information. The mixer 20 also receives the high and low frequency television synchronization pulses produced in the generator 9 and acts to mix them, in any suitable manner with the signals representing trafiic information, for example, the pulses representing traffic information may be positive-going pulses Whereas the synchronization pulses may be negative-going. Along with the traic information signals from the group of translators the mixer 20 receives the timing mark produced by the pulse generator associated with the commutator device.

Block 2I represents a generator of television signals representing the "self-identification or marker line. It may be a generator of the kind shown in the Jones patent application Serial No. 607,9991/2. Whether it is of that type, or is of a type employing a cathode ray picture tube scanned in accordance with the pulsing of the radar and the rotation of its directional antenna to produce a radial strobe line angularly rotating about one of its ends, it is necessary that an element of the generator (block 2|) be controlled in accordance with the radar antenna rotation. Accordingly, the mechanical link I 'I is shown also to be connected to the generator 2| the connection shown involving a second branch link 22. The output of block 2 I, which will be in the form of video pulses Will also be fed to the mixer 2G. These pulses are adjusted to be of substantially smaller amplitude than the pulses representing the traiiic information which are delivered overoutput conductor I9.

Mixer 2E) is connected to a television transmitter 23 so as to modulate its carrier wave out-- put with the composite video signal produced in the mixer. The high frequency television signals are broadcast over an antenna 24 and, as represented in the drawing by an arrow 25, they are intercepted by an antenna'26 of the airborne television receiver carried in the same craft as is carrying the beacon 2. As previously was mentioned in referring to the beacon 2, under normal conditions there usually will be a plurality of aircraft in the operational area served by the aircraft. Accordingly there will be a plurality of television receivers like the one shown in the single example herein. For this and other 9 actuated by the gate pulses produced in the gate generator 43.

Each time that the radar l interrogates the beacon 2, gated amplifier 50 will transfer medium level pulses to the coupling circuit 4B over which they will reach the indicator 49. This will cause the appearance of the self-identification line on the plan-position display produced by the indicator 49. Since this line will occur in the display at an angle which corresponds to the azimuthal bearing of the antenna 3 at the time of the interrogation, it will pass through the one pip on that display which represents the craft carrying the beacon 2.

For the purpose of producing a television type of raster on the iiuorescent screen of the picture tube of the indicator 49 the sweep generators 34 are connected to appropriate beam deflection elements of the indicator 49 over circuits 5I and 52.

The third object of this invention is attained due to the fact that the gating circuit for causing the self-identification line to be displayed does not act upon the channel of the television receiver which is carrying high-level signals representing the various-subdivisions of traffic information and that therefore it cannot cause high level signals in a Wrong time channel to be applied to the indicator 49 to display information which is intended to be suppressed.

I claim as my invention:

1. A receiving system comprising a television receiver for receiving composite television signals including first pulses representing traffic information. second pulses representing a selfidentiiication line, and third pulses for synchronizing the sweep generators of the receiver, first, second, and third separate video frequency signal circuits in said receiver, means in the receiver for separating the three kinds of pulses to apply them respectively to the separate signal circuits, an indicator responsive to the first pulses to produce a display of traflic information including visible spots representing the positions of craft and responsive to the second pulses to produce an identification display identifying one of the spots as representing a particular craft. said iirst and second signal circuits being both connected to said indicator for applying thereto said first and second signals. a normally closed gate in said second signal circuit, a pulse receiver associated with the television receiver, a gating pulse generator connected to the output of the pulse receiver and responsive to a train of short-duration pulses to produce a long gating pulse, and means for applying the gating pulse to the gate to open it for the transmission of said second pulses to said indicator.

2. A receiving system as in claim 1 also comprising in the connection between said pulse receiver and said gating pulse generator a coincidence circuit for selectively responding to pulses from the former to provide pulses to the latter, said third signal circuit being connected to the coincidence circuit for applying thereto said third pulses to cause the coincidence circuit to respond to any pulses from the pulse receiver which are respectively in a synchronism with ones of said third pulses whereby the gating pulse generator will produce a gating pulse only when said third pulses in a composite signal received by the television receiver are in synchronism with pulses simultaneously received by the pulse receiver.

3. A receiving system as in claim 1 also comprising a commutator device adjustable to produce a gate pulse extending over any selected one of a group of sequentially occurring predetermined time channels, said first signal circuit including a normally-open gate, and means for controlling the gate with said gate pulse to transfer said first pulses to the indicator during any one of said time channels.

4. A receiving system as in claim 1 also comprising in the connection between said pulse receiver and saidgating pulse generator a coincidence circuit for selectively responding to pulses from the former to provide pulses to the latter, said third signal circuit being connected to the coincidence circuit for applying thereto said third pulses to cause the coincidence circuit to respond to any pulses from the pulse receiver which are respectively in a synchronism with ones of said third pulses, a commutator device adjustable to produce a gate pulse extending over any selected one of a group of sequentially occurring predetermined time channels, said iirst signal circuit including a normally-open gate, and means for controlling the gate with said gate pulse to transfer` said first pulses to the indicator during any one of said time channels.

5. In a radio navigation system comprising a ground station including a radar which has a pulse modulated transmitter and an angularly scannable directive antenna and is operable for obtaining and displaying signal information as to the positions of mobile craft in an operating area and a television transmitter responsive to said radar display for broadcasting the signal information to mobile craft in the area. and at least one mobile receiving station which is carried in a craft and includes a radar pulse receiver, a television receiver and a television indicator responsive to output from the television receiver to produce a display of traflic information including visible spots representing the positions of craft, in which the system further comprises means at the ground sta-tion for causing each of the transmitted radar pulses to occur in synchronism with one of the television synchronization pulses broadcast from the television transmitter, a coincidence circuit carried in the mobile craft and associated with both the radar pulse receiver and the television receiver, means in the mobile craft for applying received radar pulses and television synchronization pulses to the coincidence circuit, the coincidence circuit being responsive to a radar pulse when it is applied thereto in synchronism with one of the television synchronization pulses to produce an output voltage, a gate in the television receiver responsive to said output voltage so to increase what part of the output of the receiver is applied to said indicator as to produce an identification display, and means for applying said output voltage to the gate for causing it to respond whenever said radar pulse receiver and said television receiver both simultaneously are receiving transmissions from said ground station.

6. In a radio navigation system as in claim 5 in which the composite signals broadcast by said television transmitter include first pulses representing trafiic information, second pulses representing said identification display and third pulses for synchronizing the sweep generators of said television receiver, rst, second and third separate video frequency signal circuits in said television receiver, means in said television receiver for separating the three kinds of pulses to apply them respectively to the separate signal circuits, said indicator being responsive to the first pulses einsame A ,.tofproducesaididisplay.oitrafcdnformationand responsivewtowsaidcsecond pulses toprodncasaid identification display, and saidgate is. vcorn'lectecl seresdni-,said y.second-i signal. circuit.

.-In Yaradio .-nav-igatiomsystem. .as .in =-cla.irn 6 iwhich also., comprises in .the mobile receiving` station a commutator 'deviceadjnstabk-to `produce ae lgate .zpulsel-.extending. over any.. selected one of a.group of sequentially. occurring predetermined .time channels,said..rst .signal circuitincluding a...normallyfopengate and..means f or controlling the\ normally-openf.gate ,witliisaidhgate pulsey Ase- .lectivelyftmtransferr said .first .pulses .to .the indicator l during... any .one-.oil saidA time.. channels.

8. rApparatusto.lcelcarried-.by a.,mobi1e. craft .comprsingin ,combinaomatelevision receiver forreceiving.. identification dsplayproducing `sigr .nalsf i and.. accompanying, ...,synchronizing.. signals, said receiver including amindicator. display de- `vice; vsaid receiver, .also including .mansior separating; said identicatiomdisplay yproducingpsig.- nals andsaid..synchr.onzing signals--fromach other, a.` transponder b eacontlxat `includes .a.bea con receiver .for receiving radarrpulses .transmitted from a.radar ground ,station in.synchro; nism.witlnsad...Synchmnisnnpulses;v said-f tele- -vision.-.. receiver,f..including annormally blocked channel for supplying said identification-.display .producingsignalslta said. indicator. displayrdevice,

and .meansnineludingncoincidence. circuit..for .unblocki'ng .said,..channe1 in. response to the .application,tsaidcoincidence oircuitof ,bothradar p11lses fwmfe'saidbcacond receiver andV television synchronizing; -.pu1ses.,..from saidi television .re- ,ceiven 

